This invention relates to numerically controlled machine tools. It is known in such machine tools to move a tool into engagement with a workpiece under the control of a closed loop system having a feedback signal defining the distance between the tool and a datum surface provided on a fixed part of the machine.
It is the practice to move the tool initially at a relatively high speed so as not to waste time and to reduce that speed when the tool comes close to the workpiece so as to avoid undue impact forces between the tool and the workpiece. If the position of the tool surface to be engaged is not accurately known the speed change has to be made at a relatively early stage so that the time of a slow tool speed could be relatively long. It is an object of this invention to overcome or reduce this difficulty.